The present invention relates in general to automatic gain control in an intermediate frequency section of an AM receiver, and more specifically to providing automatic gain control with improved response time, so as to avoid overshoot or undershoot in AGC performance.
Automatic gain control (AGC) is a well-known technique for maintaining a substantially constant average level of an amplified output signal even though the input signal level is varying. In broadcast communication receivers, automatic gain control is employed to provide a consistent audio output level despite fluctuations in signal strength of a tuned-in broadcast signal or fluctuations occurring when retuning to other broadcast signals of different power levels or distance from the receiver.
An AGC circuit controls the gain of an amplifier stage in inverse proportion to a measured average signal level downstream of the amplifier stage in the receiver. In an amplitude modulated (AM) receiver, the information signal consists of amplitude variations in a transmitted carrier signal. The information signal creates sidebands around the carrier frequency. The measured signal for determining average signal level to provide AGC control in prior art receivers has been the AM intermediate frequency (IF) signal including the carrier signal and sidebands. Therefore, AGC circuits in prior art AM receivers must be designed so that they will not respond to amplitude variations corresponding to the information signal. Typically, the AM IF signal is lowpass filtered with a long time constant to remove any audio information signal prior to an AGC control circuit. However, the lowpass filtering results in a long response time of the AGC control circuit to sudden variations in the average signal level.
Sudden variations in signal level may occur when a vehicle carrying an automotive radio receiver moves from an open environment into a tunnel or under an underpass or viaduct, for example. Although the signal strength of the AM signal may quickly decrease, the AGC control circuit responds to the change only after a delay, resulting in a decreased audio output (i.e., AGC undershoot) of the received AM station and an increase in noise during the delay. Likewise, when the vehicle reemerges from a tunnel or viaduct and the signal blockage is removed, an overamplified AM IF signal (i.e., AGC overshoot) can be produced.
These are some of the disadvantages that the present invention overcomes.